1. The Field of the Invention
The invention generally relates to vertical cavity surface emitting lasers (VCSELs) and photodiodes. More particularly, the invention relates to VCSELs and photodiodes fabricated in a monolithic structure.
2. Description of the Related Art
Lasers have become useful devices with applications ranging from simple laser pointers that output a laser beam for directing attention, to high-speed modulated lasers useful for transmitting high-speed digital data over long distances. Several different types of lasers exist and find usefulness in applications at the present time. One type of laser is the edge emitter laser which is formed by cleaving a diode from a semiconductor wafer. Cleaving a diode from a semiconductor wafer forms mirrors that form a laser cavity defined by the edges of the laser diode. Edge emitter lasers may be designed to emit a laser beam more strongly from one of the edges than the other edges. However, some laser energy will be emitted at the other edges. Edge emitter lasers are commonly used when high optical power is needed.
A second type of commonly used laser is known as a vertical cavity surface emitting laser (VCSEL). A VCSEL is formed in part by forming a first mirror from Distributed Bragg Reflector (DBR) semiconductor layers. The DBR layers alternate high and low refractive indices so as to create the mirror effect. An active layer is then formed on the first mirror. A second mirror is formed on the active layer using more DBR semiconductor layers. Thus the VCSEL laser cavity is defined by top and bottom mirrors which causes a laser beam to be emitted from the surface of the laser. Laser diodes generally operate using a forward bias. To forward bias a laser diode, a voltage is applied to the anode and a lower voltage or ground is connected to the cathode.
In some simple applications, the lasers may be operated open loop. I.e., the lasers do not require feedback, or can operate satisfactorily without feedback. For example, in most laser pointer applications, the output power of the laser beam may be controlled without reference to the actual output power. In other applications, it may be very important to precisely gauge the amount of output power emitted by the laser while it is operating. For example, in communications applications it may be useful to know the output power of the laser such that the output power of a laser may be adjusted to comply with various standards or other requirements.
Many applications use a laser in combination with a photodiode or other photosensitive device to control the output of the laser. A photodiode has a current output that is proportional to light impinging on the diode. The photodiode either has no bias or is implemented in a reverse bias configuration such that the cathode is connected to a high voltage while the anode is connected to a low voltage or ground. In a photodiode in the reverse biased or unbiased configuration, current is generated within the photodiode as light impinges the photodiode.
An appropriately placed photodiode may be used as one element in the feedback circuit for controlling the laser. For example, a photodiode used in an edge emitter laser application may be placed on one of the two active edges of the edge emitter laser diode. While the power output at each of the active edges may be different, the power at each active edge is proportional by some factor to the power output at the other active edges on an edge emitter laser diode. Similarly, the photodiode may be used to monitor the output power from other types of lasers.
Optical output is normally available only from one surface of a VCSEL. Thus, the use of a photodiode to monitor the VCSEL output requires that a portion of the optical output from the VCSEL be diverted so as to impinge on the photodiode. For example, this function may be implemented by reflecting light from the exit window on the VCSEL package. The exit window may be flat or slanted. With a flat window package the VCSEL may be mounted on top of the monitor photodiode. With a slant window package the VCSEL and photodiode are mounted side by side. However, accurate control of the angle and reflectivity of the slanted window and accurate placement of the two devices are important to achieve predictable coupling. This requirement for precise control of both package and mounting tolerance of the VCSEL and photodiode results in an increase in cost.
When using a monitor photodiode it is important that the photodiode current be proportional to the output power from the VCSEL that is coupled to the optical fiber for all bias currents and over temperature. This ratio of photodiode current to coupled power is called tracking ratio.
Various challenges exist when implementing a laser diode and photodiode together. While the laser diode and photodiode are both formed from semiconductors and use similar construction techniques, they are often made from different materials and they have generally been implemented as separate devices. This necessitates a different power supply be used for biasing the laser diode and photodiode when they are biased using opposite polarities. Using two discrete components, further, results in an increase of cost.
Attempts have been made to integrate the laser diode and photodiode monolithically on a single wafer substrate. However, this may still require the use of different power supplies. For example, in the case when the laser diode and photodiode share a common cathode or anode. Additionally, the photodiode may be placed on top of the VCSEL or within a mirror that is part of the VCSEL. This however has the unfortunate drawback of causing the photodiode to become a part of the optics, particularly the mirror, of the laser thus altering the optical characteristics of the laser.
Therefore, what would be advantageous are mechanisms for implementing laser diodes and photodiodes monolithically using a single power supply.